Scientists have debated why the several species of moa went extinct about 100 years after Polynesians settled New Zealand around A.D. 1300. Various lines of evidence suggest that people’s hunting, setting fires and bringing competing species to the islands caused the big birds’ demise. But recent genetic evidence hinted that moa were declining before the Polynesians ever reached New Zealand. The birds may have been victims of disease and volcanic eruptions that reduced their numbers and genetic diversity.

Now, an international group of researchers has analyzed more complete DNA records from fossils of four species of moa. The fossils range in age from around 600 to 13,000 years old. The researchers saw no evidence of decline in the birds’ genetic diversity over the last 4,000 years of their existence. In fact, the population of one species, Dinornis robustus, increased, the team reports March 17 in the Proceedings of the National Academy of Sciences. The new findings point to humans as the reason moa went extinct.